While fans were sad that The Big Bang Theory had aired what seemed like their last season, they were presented with one of the best finales ever written. When they found out it was the last season, everyone involved wanted to make it a moment to remember forever. Fans particularly loved when Jim Parsons saw Sarah Michelle Gellar in the audience for the first time. Still, few knew that the Sheldon actor's reaction to seeing her was genuine.
Although it wasn't officially confirmed at the time of airing, Season 12 of The Big Bang Theory proved to be the last. When it ended, there were no plans for the next season.
Most of the cast quietly hoped another season was in the works. However, Jim Parsons (who played Sheldon throughout the series) was content with the performance that he had given and decided that the twelfth season would be his last.
Ending his contract after the finale, and without Sheldon, The Big Bang Theory had no choice but to wrap up the show, and showrunners needed to come up with the best plan to make it happen.
To make the finale as memorable as possible, showrunner Steve Holland and director Chuck Lorre had a perfect plan to make the final episode memorable for the cast and fans watching around the world.
It's impossible to argue that the major highlight of the final episode of The Big Bang Theory was Sheldon's speech as he accepted his Nobel prize, something that he has been working on and hoping for the entire show and his entire life.
As he stands there, Sheldon gives an emotional speech, thanking his friends in the audience (and Amy beside him with her tiara) for their contributions over the years as friends and colleagues.
During his emotional speech, there is a moment when Sheldon spots who is sitting next to Raj in the audience, and it is none other than Sarah Michelle Gellar. In a stunned moment, Parsons says, "Is that Buffy the vampire slayer?" in the middle of his epic Nobel speech.
Few people realise that his reaction was entirely genuine and was so perfect that they simply had to use it in the final cut.
Apparently, it wasn't difficult to convince Gellar to participate in the final episode because her family absolutely loved the show, so she instantly agreed to participate. It was a no-brainer, as many of the cast loved Buffy the Vampire Slayer, but not nearly as much as those in the writer's room. For everyone involved, having her on the show was a big win and surprise, and it's charming to know that Sheldon's reaction was genuine.